Obey absolute letterSpacing when writing PDF.

* src/qt/src/gui/painting/qpdf.cpp
  (QPdfBaseEnginePrivate::drawTextItem):

  Positions of each glyph did not take into account the letterSpacing
  property of the text item's QFont.  Here we adjust using the
  absolute spacing, if it is present, via a not-so-pleasant method.

  This makes absolute settings of the CSS letter-spacing property
  actually apply in non-SVG contexts when writing PDF.  Spacing is
  already handled correctly for SVG, because qpdf isn't responsible
  for layout of individual glyphs in that case.

  Relative letter-spacing and any word-spacing are still not
  supported.
Integrate pdf-custom-letter-spacing patch (2f5f2bf1).